Monday, August 29, 2011

The red line in this image shows the long-term increase in global sea level since satellite altimeters began measuring it in the early 1990s.Since then, sea level has risen by a little more than an inch each decade, or about 3 millimeters per year.While most years have recorded a rise in global sea level, the recent drop of nearly a quarter of an inch, or half a centimeter, is attributable to the switch from El Niño to La Niña conditions in the Pacific.The insets show sea level changes in the Pacific Ocean caused by the recent El Niño and La Niña. Image credit: S. Nerem, University of Colorado

Earth's seas have been rising gradually with the influence of climate change, but over the past year, this rise hit something of a pothole, with a temporary dip linked to the El Niño-La Niña cycle and all the rains that have poured onto the planet's continents.

For the past 18 years, the U.S./French Jason-1, Jason-2 and Topex/Poseidon spacecraft have been monitoring the gradual rise of the world's oceans in response to global warming.
This rise is caused both by the expansion of the ocean's waters as they warm up, and by the melting of glaciers and ice sheets.

Most of the time, this rise is a steady one, but between last summer and this one, global sea level actually fell by about a quarter of an inch (about half a centimeter).

El Niño, La Niña

The shift from an El Niño pattern (where waters over the eastern Pacific Ocean are warmer than normal) to a La Niña pattern (where the same waters are cooler than normal) is likely behind the dip, as it altered global rainfall patterns, said climate scientist Josh Willis of NASA's Jet Propulsion Laboratory (JPL) in Pasadena, Calif.

Data from the NASA/German Aerospace Center's twin Gravity Recovery and Climate Experiment (GRACE) spacecraft provide a clear picture of how this extra rain piled onto the continents in early 2011.
"By detecting where water is on the continents, GRACE shows us how water moves around the planet," said Steve Nerem, a sea level scientist at the University of Colorado in Boulder.

Gravity Recovery and Climate Experiment (Grace) map shows how much water was lost or gained over the continents between the spring of 2010 and the spring of 2011.The red colors show dry regions where water was lost.The blue colors show places that gained water, usually because of heavier-than-normal rainfall or snow.Image credit: NASA/JPL-Caltech

Seas still rising

All the extra water that piled onto the continents came from — that's right — the ocean. Each year, huge amounts of water are evaporated from the ocean.
While most of it falls right back into the ocean as rain, some of it falls over land.

"This year, the continents got an extra dose of rain, so much so that global sea levels actually fell over most of the last year," said Carmen Boening, a JPL oceanographer and climate scientist.
Boening and colleagues presented these results recently at the annual GRACE Science Team Meeting in Austin, Texas.

Of course, this sea level dip is just a small blip in the overriding trend of sea level rise, Boening said.
Water flows downhill, and the extra rain will eventually find its way back to the sea.
When it does, global sea level will rise again.

"We're heating up the planet, and in the end that means more sea level rise," Willis said. "But El Niño and La Niña always take us on a rainfall roller coaster, and in years like this they give us sea-level whiplash."